Thick matter pumps typically have two feed pistons which are operated in a push-pull manner by hydraulic drive cylinders, and which lead into a material charge vessel. Each feed piston is coupled by a common piston rod with a respective drive piston of the respective drive cylinder, and their vessel-side openings are connectable with a feed line by means of a pipe switch. The pipe switch is pivotable by at least one hydro-cylinder, during the pressure stroke, and is open toward the interior of the vessel during the suction stroke.
At one end, the drive cylinders are pressurized alternately with high and low pressure by means of a hydro-pump, based on the position of a switching valve. The other ends of the drive cylinders are coupled together hydraulically. However, when the end positions of the pistons are reached in the feed cylinders and/or in the drive cylinders, the switching valve is actuated. The delivery of hydraulic fluid to the drive cylinders and to the hydro-cylinder actuating the pipe switch is simultaneous.
Known pumps also include a pressure equalizing line, which includes a check valve, and is coupled to both ends of one of the two drive cylinders in order to correct the stroke thereof. With such stroke correction, the two drive cylinders can be operated synchronously, despite inevitable leakage from the high-pressure to the low-pressure side of the drive cylinder pistons.
For actuating the switching valve, known pumps also provide on the rod-side end of the feed cylinder, in the region of a water box, an electric switching element. The electric switching element furnishes a switching pulse when the feed piston reaches its end position in the water box and, thus, when the respective drive cylinder reaches its bottom end. To obtain a reliable correction of the stroke, the switching contact must be arranged so that when located in its end positions, the piston sufficiently sweeps the length of the pressure equalizing line.
With this type of switching element, however, the leakage and, thus, the correction range depends on the stroke velocity. As a result, the variation in feed quantity is restricted within narrow limits. Also, to be able to satisfy the stringent requirements for feed quantity variation in industrial installations, hydraulic signal sensing on one of the cylinders has been provided to ensure stroke correction independent of the chosen stroke velocity.
With two pressure switching valves located on one drive cylinder on the bottom and rod sides thereof, and with two stroke correction lines on the other drive cylinder, the synchronism of the two cylinders after every second stroke is typically ensured. This is true for bottom-side as well as for rod-side drives With this type of hydraulic signal sensing, however, in the no-load case (i.e., when operating without load or at low pump resistance), the pressure necessary for reversing the pressure switching valves, or the pressure build-up necessary for switching, is reached only in the end position of the drive piston, due to the differential ratio of the drive cylinder. In no-load operation, this leads to the piston striking the cylinder wall in the end position.
It is an object of the present invention, therefore, to provide a control system for two-cylinder thick matter pumps in which a large feed quantity variation is possible and yet a stroke correction without striking the piston against a cylinder wall is ensured.